Field of the Invention
The invention relates to an optical system of a microlithographic projection exposure apparatus.
Prior Art
Microlithography is used for producing microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. In this case, the image of a mask (=reticle) illuminated via the illumination device is projected, via the projection lens, onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process.
During the operation of a projection exposure apparatus, there is a need to set, in the illumination device, for the purpose of optimizing the imaging contrast, in a targeted manner, specific polarization distributions in the pupil plane and/or in the reticle and also to be able to make a change to the polarization distribution during the operation of the projection exposure apparatus.
In this case, one problem which occurs in EUV systems, in particular, is the increasing transmission loss associated with the reflections required for this purpose, as a result of which the performance properties of the projection exposure apparatus are impaired.
With regard to the prior art with respect to changing the polarization distribution in projection exposure apparatuses designed for the EUV range, reference is made merely by way of example to DE 10 2008 002 749 A1, US 2008/0192225 A1, WO 2006/111319 A2 and U.S. Pat. No. 6,999,172 B2.